The invention relates to a data transmission method used in a radio system comprising at least one base station and a subscriber terminal which modulate a signal to be-transmitted on subcarriers of a multicarrier modulation and transmit a reference signal in a band in use by using reference carriers formed by the multicarrier modulation.
The invention further relates to a radio system comprising at least one base station and a subscriber terminal comprising a transmitter and a receiver arranged to multicarrier modulate a signal to be transmitted on subcarriers and to transmit and receive a reference signal which is multicarrier modulated on reference carriers.
In an OFDMA method (Orthogonal Frequency Division Multiple Access) a spread-coded string of symbols of a signal to be transmitted is modulated on subcarriers which are preferably distributed into a broad frequency band. OFDMA modulation is typically conducted by performing an inverse Fourier transform.
In prior art solutions a pilot signal is also transmitted in the radio system using reference carriers by which, for example, synchronization is facilitated and a channel estimate is formed. The reference carriers are added to the frequency band of the data to be transmitted. A predetermined symbol or sequence of symbols, whose number of symbols corresponds to the number of subcarriers, is transmitted on the reference carriers. Each subcarrier transmits the same reference symbol in each time slot which makes it easy to distinguish the reference symbols from the OFDM symbols of simultaneously transmitted data.
Transmitting and receiving the OFDMA signal allows the use of differential coding and detection, whereby there is no need to form a channel estimate. The performance of differential detection is, however, inferior to that of coherent detection in which the locking takes place into the phase of a received signal.
In coherent reception a channel estimate can be formed for each subcarrier of a data signal by interpolating at time and frequency level the reference carrier transmitted in the immediate surroundings of the subcarrier. Such a local interpolation to form the channel estimate, however, significantly degrades a bit error ratio compared with an ideal channel estimation. When the channel features slowly change compared with the duration of the data symbol, the decisions made from previous symbols can also be utilized when the channel estimate is formed. However, the channel estimator operating by decision feedback is not possible because the estimator needs initial values for estimates in order for the decision-making to be meaningful. Furthermore, the channel estimator cannot be applied to systems using frequency hopping.
An object of the present invention is to implement a coherent signal reception in a radio system using multicarrier modulation, in which radio system a channel estimate is formed without interpolation or estimation from previous symbols. Another object is to enable coherent reception also in a system using frequency hopping.
This is achieved with a method of the type set forth in the preamble, characterized in that reference carriers are substantially randomly placed in a band in use.
A radio system of the invention, characterized in that a transmitter of the radio system is arranged to substantially randomly place reference carriers into a band in use.
Great advantages are achieved with the method of the invention. The space of a data transmission channel can be accurately estimated enabling the use of coherent detection at a receiver.